1. Field of the Invention
The present invention relates to a structure of a contact portion to form a planarized conductive film in a semiconductor device using a thin film. Specifically, the present invention relates to a structure of a contact portion to contact a switching element and a pixel electrode electrically in a pixel region of an active matrix-type liquid crystal display (hereinafter referred to as AMLCD).
2. Description of the Related Art
Recently, technique of fabricating a TFT on an inexpensive glass substrate has been developing with increased demand for AMLCD (Active Matrix-type Liquid Crystal Display).
An AMLCD in which a thin film transistor (TFT) as a switching element is disposed in each of half a million to multimillion pixels arranged in matrix respectively and electric charge which moves in and out each pixel electrode is controlled by switching means of the TFT.
A liquid crystal is sandwiched between each pixel electrode and the opposite electrode to form a kind of a capacitor. Therefore the strength of electric field applied to the liquid crystal is regulated by controlling the amount of electric charge which moves in and out of the capacitor. In this way, a pixel display can be realized by controlling amount of light which transmits a liquid crystal display device according to an electro-optics characteristic of the liquid crystal display device. In addition, an image display can be performed at large by individually controlling the amount of light in each pixel of a panel.
There is a characteristic phenomenon of a display device using such a liquid crystal, which is called a light leakage. A liquid crystal is arranged between a pixel electrode and the opposite electrode with a regular orientation. The alignment disturbance of the liquid crystal is caused due to an inferior rubbing for pattern steps, unevenness of a pixel electrode surface, and a distortion of electric field applied to a liquid crystal when a liquid crystal display device is driven. A light leakage is to have higher light transmittance in the parts where alignment disturbance of a liquid crystal is caused. Specially, in a liquid crystal display device of projection formula, the display grade is widely degraded by a light leakage. In this case, it is difficult to characterize electro-optically as it is to be in the parts where alignment disturbance of a liquid crystal is caused. Typically, a pixel displaying a gradation sequence close to a black level has higher transmittance than the pixel displaying a gradation sequence that should have due to the influence of a light leakage, and results in difficulty in displaying a sufficient black color. As a result, the degradation of display grade and deterioration of contrast are generated.
Previously, a device such as providing a structure in which a TFT is covered by a planarizing film has been made to prevent disclination, it does not become a essential solution in the present condition because of the difficulty in planarization of steps at the contact portion in a pixel electrode which is formed lastly no matter how a planarizing film is used.
FIG. 10 is a sectional pattern view showing a conventional semiconductor device. A drain electrode 101 composed of a metal electrode is formed on a base film which is not shown in the drawing, and an interlayer insulating film 102 is formed thereon. Then, a drain contact hole 102a located on a drain electrode 101 in the interlayer insulating film is formed by providing a resist film on the interlayer insulating film 102, and dry etching the interlayer insulating film 102 by using the resist film as a mask. Next, a pixel electrode 103 connected to a drain electrode 101 in a drain contact hole 102a is formed on the interlayer insulating film 102 by forming a conductive film in the drain contact hole and on the interlayer insulating film and patterning the conductive film. In addition, an overlap region 103a is formed on the pixel electrode 103 to cover the whole inside of a drain contact hole 102 with the pixel electrode 103.
In regard to the planarization of a TFT substrate surface, measures are taken against the opening region and the wiring vicinity, the steps of the drain contact hole to provide continuity across the pixel electrode and the TFT is left as it is as shown in FIG. 10. These steps are expected to cause alignment disturbance of liquid crystal and bad influence on rubbing process of the following processes.